how to solve Error cannot add duplicate collection entry of type add with unique key attribute ‘value’ in iis 7

IIS7 defines a defaultDocument section in its configuration files which can be found in the %WinDir%\System32\InetSrv\Config folder. Most likely, the file index.aspx is already defined as a default document in one of IIS7’s configuration files and you are adding it again in your web.config.

I suspect that removing the line
<add value="index.aspx" />

from the defaultDocument/files section will fix your issue.

The defaultDocument section of your config will look like:

<defaultDocument>
  <files>
    <remove value="default.aspx" />
    <remove value="index.html" />
    <remove value="iisstart.htm" />
    <remove value="index.htm" />
    <remove value="Default.asp" />
    <remove value="Default.htm" />
  </files>
</defaultDocument>

Note that index.aspx will still appear in the list of default documents for your site in the IIS manager.

For more information about IIS7 configuration, click here.